Behavioral interactions between licitly and illicitly abused drugs will be studied in human volunteers. Reliable experimental procedures have been developed for assessing cigarette smoking behavior. Self-administration behavior is monitored by counting the number of cigarettes smoked, and by measuring the frequency, size and duration of indivivual cigarette puffs. A carbon monoxide analyzer is used to provide a physiological measure of amount of cigarette smoking. In some studies volunteers live on a residential research ward on which ciagarette smoking is studied on a continuous basis for up to 6 weeks. In other studies volunteers report to the research area for daily sessions lasting from 1 to 3 hours. Experiments will be undertaken to characterize the patterns of smoking which develop under these experimental conditions when basic variables are manipulated such as cigarette dose or response requirement to obtain drug. Subsequently, studies will be conducted to explore the modulation of pattern and amount of cigarette smoking by several drugs including ethanol, pentobarbital, d-amphetamine, and caffeine. Overall, these studies should provide an improved understanding of the addictive process by describing the interactive relations among commonly abused drugs.